P5 Plant Transport System Notes

A. Overview of the Plant Transport System

The plant transport system consists of two types of tubes that carry substances to all parts of the plant:

Type of Tube	Also Known As	Function
Food-carrying tubes	Phloem tubes	Transport food made by leaves to all parts of the plant.
Water-carrying tubes	Xylem tubes	Transport water absorbed by roots to all parts of the plant.

B. Food-Carrying Tubes (Phloem)

Leaves make food in the form of sugars during photosynthesis. Some of the sugars are used by the plant for growth and other life processes.

Excess sugars are transported to other plant parts such as flower, fruit, stem, and roots for storage. The excess sugars are usually converted into starch.

Sugars from the leaves may move up or down through the stem.
Food-carrying tubes transport food in both directions — upward to fruits and downward to roots.

C. Water-Carrying Tubes (Xylem)

Water is absorbed by the roots, and the water-carrying tubes transport it to all parts of the plant.

Excess water evaporates and escapes from a plant through the stomata found on its leaves.
Mineral salts are dissolved in water and are also transported through the water-carrying tubes.
Water and mineral salts are transported up the plant from the roots.

D. Experiment: Removing Food-Carrying Tubes

Scenario 1: When food-carrying tubes are removed but water-carrying tubes remain intact:

The leaf above the cut remains healthy because water can still reach it through the water-carrying tubes.
The leaf can continue to make food through photosynthesis using the water supplied.

Scenario 2: When food-carrying tubes are completely removed around the stem:

The part above the cut becomes swollen because food made by the leaves cannot be transported downward.
Food accumulates above the cut, causing swelling.
After a few weeks, the plant dies because roots die from lack of food, and without roots, water cannot be absorbed — causing the whole plant to die.

E. Experiment: Removing Water-Carrying Tubes

Scenario 1: When water-carrying tubes are cut above a leaf:

The leaf above the cut dies because water absorbed by the roots cannot reach it.
Without water, the leaf cannot make food through photosynthesis and dies from lack of both water and food.

Scenario 2: When only part of the stem is cut (water-carrying tubes remain intact to another leaf):

The leaf below the cut remains healthy because water can still reach it through the remaining water-carrying tubes.
The leaf continues photosynthesis and stays alive.

F. Stomata in Plants

Tiny openings on plant leaves are called stomata. They serve two main functions:

Function	Description
Gaseous exchange	Allow gaseous exchange between the plant and surrounding air to occur.
Water loss	Water vapour escapes through the stomata.

G. Stomata: Day vs Night

	In the Day (Open Stoma)	At Night (Closed Stoma)
Sunlight	Present — plant undergoes photosynthesis.	Absent — photosynthesis cannot take place.
Stomata state	Open to allow gaseous exchange.	Closed to reduce water vapour loss (small gap remains for gaseous exchange).
Photosynthesis	Carbon dioxide taken in, oxygen given out.	N/A — does not occur without sunlight.
Respiration	Oxygen taken in, carbon dioxide given out.	Oxygen taken in, carbon dioxide given out (occurs all the time).

H. Key Takeaways

Phloem tubes transport food (sugars/starch) from leaves to all parts of the plant.
Xylem tubes transport water and mineral salts from roots upward through the plant.
Stomata are tiny openings on leaves that allow gaseous exchange and water vapour loss.
Plants carry out respiration all the time, but photosynthesis only occurs in the presence of sunlight.